Florida swamps - ancient sinkholes

In Florida, the limestone bedrock is protected by a surface layer of sand and clay. Thus, when a sinkhole forms naturally, over centuries, sand and clay fill the void, creating a muddy depression. These depressions tend to evolve into swamps.

When left alone for long stretches of time, swamps created by sinkholes have deepened and broadened into Florida’s signature caves, estuaries and wetlands — well-known in the Everglades but also common throughout the rest of the state, says Robert Brinkmann, a geologist at the University of South Florida in Tampa. Topographic maps of the center of the state reveal relatively dry uplands and a patchy distribution of low-lying wetlands, most of which trace their origins back to sinkholes. “Every little wetland is a little sinkhole,” Brinkmann says.

Defined loosely as any parcel of land that stays wet for extended periods of time, a wetland is entirely dependent on the groundwater level, or water table. When the water table is near the surface, such as during Florida’s rainy season, these depressions fill with water, providing a suitable habitat for the scaly, feathered and leafy beings that live in wet environments. When the water table drops and the wetlands dry out, those creatures disperse. “A very slight change in water table elevation — anything more than about a foot — and you really start to change those wetland systems,” says Mark Stewart, also a geologist at the University of South Florida.

Because of agricultural demand during January’s deep freeze event, the water table dropped close to a meter due to groundwater removal. The impact on the area’s wetlands remains unknown, Brinkmann says.